The present invention relates to a method and an assembly for robust one-shot interferometry, in particular for optical coherence tomography according to the spatial domain approach (SD-OCT) and/or according to the light-field approach. The method and the assembly can be used for measurements on material and living tissue, for distance measurement, for 2D or 3D measurement with a finely structured light source imaged onto the object in a diffraction-limited way, or with spots thereof.
The published patent application DE 10 2006 015 387 A1 [A1] by M. Hering et al. describes an interferometric measuring device on the basis of white light interferometry, also known as short-coherence interferometry, in which the wavefronts of the reflected object beam and those of the reflected reference beam are inclined with respect to each other by a specific angular magnitude by means of an inclination device, such that a spatial interferogram can form as a single one-shot data set. For example, this angular magnitude is realized here in a strongly modified Linnik interferometer arrangement, which also exhibits features of a Mach-Zehnder interferometer, by means of a tilting mirror through which light passes only once on the path to detection. With this optical assembly, it is possible to fully provide one or more spatial interferograms, also as line stacks on a matrix camera, as single-shot data sets in the time period of image acquisition.
It is of particular advantage with this approach that the spatial frequency for the main wavelength, or main wavenumber, in the spatial interferogram at the output of the interferometer is, in a first approximation, not influenced by the inclination of the object surface in relation to the interferometer.
A targeted change in the angular magnitude by means of an inclination device, for example in order to change the spatial frequency for the main wavelength in the spatial interferogram in a predetermined way, may lead to an undesired lateral offset of object wavefront and reference wavefront during detection, which can be compensated for only in a complex manner or cannot be compensated for at all by an alignment in some cases, and may constitute a source of measurement errors or limit the depth measurement range considerably. As a rule, the interferometer is not very stable in the long term.
In the publication by M. Hering et al. in Applied Optics, vol. 48, no. 3, pages 525 to 538 of Jan. 20, 2009 [2], the measured spatial interferograms in image 3 show the potential of this approach according to [1]. The one-shot interferometer measurement assembly illustrated in image 1 represents an experimental setup for purposes of study and is rather too complex and too voluminous to be realized economically. Typical measurement results on the basis of this approach were shown by M. Hering et al. already in 2006 in the Proceedings of SPIE, vol. 6188, 61880E-61880E-1 to 61880E-11 in FIG. 7 [3]. By contrast, Michelson-type interferometers having a plane reference mirror, in which the spatial frequency for the main wavelength in a spatial interferogram the output of the interferometer is to be changed by tilting the reference mirror or by tilting the object with respect to the interferometer or the interferometer with respect to the object, are not of interest here for measurement objects having a varying and unknown surface inclination with regard to the evaluation of spatial interferograms. Therefore, such approaches are not considered relevant prior art for this invention and are therefore not dealt with here any further.
Obtaining spatial interferograms for the one-shot measurement technique by means of lateral shear between object and reference wavefronts at the output of a two-beam interferometer is basically a further possibility of generating spatial interferograms for the one-shot measurement technique, for example for detecting distance. In an optical assembly, lateral shear can be used as a basis for generating interferences of mutually tilted wavefronts. A classical approach to this is a Michelson interferometer arrangement having two roof edge reflectors and a laterally extending light source to generate the required lateral shear. This approach with two roof edge reflectors is well known to those skilled in the art, cf. D. Malacara, Optical Shop Testing, John Wiley & Sons, Inc., 1992, pages. 140 to 141, FIG. 4.16 [4] and W. H. Steel, Interferometry, Cambridge University Press, 1967, p. 83 last paragraph to top of p. 84 [5].
In order to be able to use this interferometer approach with two roof edge reflectors for distance measurement and profile measurement, an additional plane mirror needs to be assigned to the object surface in the object arm of the interferometer, wherein this plane mirror together with the object surface then forms a roof edge. Here, the second roof edge reflector is arranged in the reference arm. This assembly, with a corresponding alignment, yields lateral shear between the wavefronts and avoids wavefront inversion, but generally has clear disadvantages owing to the required construction volume in the object arm, for example in the case of measurements in interior spaces.
Also known is the approach published by D. Kelsall in 1959 in Proc. Phys. Society, 73, p. 470, FIG. 1, with two triple reflectors as end reflectors of a Michelson interferometer. The transverse shift of a triple reflector also generates a lateral shear between object and reference wavefronts at the output of a Michelson interferometer. The use of a triple reflector in the reference arm of a Michelson interferometer is, to the best of our knowledge, already known from F. Twyman and A. Green, see also U.S. Pat. No. 1,565,533, FIG. 6, [7].
Using this interferometer approach as an arrangement for an interferometric sensor for distance measurement, inter alia, in which a plane mirror of the triple reflector in the form of a cube corner, also known as corner cube, is replaced by the object surface, has the effect that it is also necessary to assign a roof edge reflector or two plane mirrors to the object or to the object surface in the object arm, as the undesired wavefront inversion between object and reference optical path must be avoided for wide-area measurement, since otherwise the interference contrast will be zero. This approach enlarges the sensor volume considerably, which is very disadvantageous for many applications or entire excludes the use of such a solution.
Document DE 10 2010 006 239 B3 [8] describes an approach in which the use of a triple mirror as an end mirror in the reference arm of the interferometer entails the disadvantage that no focused bundle with a very high numeric aperture can be guided via this reference arm, which is why the measurement range may be limited, since with a high NA, e.g. above 0.7, the light returning from the reference arm is slightly limited in the aperture angle and thus is not able to have the full angular spectrum compared to the object light. This can limit the lateral range of the formation of evaluable interferences on a camera chip noticeably.
The methods and assembly according to DE 10 2010 046 907 B4 [9] have the advantage of using a second interferometer output, and the object points can be arranged almost laterally in an arbitrary manner. Assemblies according to [9] are highly stable with regard to the interferometer alignment state due to the triple reflector in the reference arm, but are also quite complicated in terms of optical circuitry. In addition, in this method, there may be problems with a spherical phase term in the wavelet if the measurement field is extensive, since the effective mirror planes in the reference and object arms do not necessarily coincide, so that a greater number of interferences with the same inclination, also known as Haidinger's rings, may form in the field. This may lead to a violation of the sampling theorem in the detection.
A method according to DE 268771 A1 [10] allows the time-resolved object detection. This approach does rather not allow shot-coherence interferometry for individual measurement points or along individual measurement lines and requires a substantial sensor volume owing to the very high lateral shear.
In Proc. of SPIE 7389, 73891J1 to -73891J12, 2009, [11] and also in PCT document WO2010/139764 A1 [12], K. Gastinger et al. describe the use of micro Mirau interferometers and Twyman-Green interferometers in array form also with a short-coherence light source for parallelized inspection of MEMS and MOEMS. For short-coherence technique, however, the optical path length over time is scanned, so that this is not a one-shot method here. Even moderate vibrations in the surrounding are highly detrimental to wavelet signals to be detected in the scan, according to image 9 at the top in [9], which in the extreme case cannot be evaluated with conventional algorithms any more or yield heavily distorted measurement results.
Already in 1996 did J. Schwider, in DE 196 32 594 A1 [13], suggest a Mach-Zehnder interferometer or a Michelson interferometer with a micro-optical array in the form of a microlens array in the object optical path for the purpose of confocal illumination of the object and confocal discrimination, for two-beam interferometry.
To the best of our knowledge, W. Emer and J. Schwider were the first to describe the use of a Schwarzschild mirror objective in a Mirau arrangement for phase-shift interferometry in the UV range in Applied Optics, 38, no. 16, pp. 3516-3522, 1999 [4]. Here, the optical path difference is varied time-sequentially in order to be able to apply the phase-shift method. Thus, it is neither a one-shot nor a short-coherence method.
In the essay “Holography Viewed from the Perspective of the Light-field Camera” Goodman [15], Joseph W. Goodman describes approaches for Fourier and Fresnel holography with a mask in the image plane. These are derived for the holography by the light-field camera. This essay can be found in the conference documents for Fringe 2013, pp. 3 to 15, 7th International Workshop on Advanced Optical Imaging an Metrology, editor Wolfgang Osten, ISBN 978-3-642-36358-0 ISBN 978-3-642-36359-7 (eBook), DOI 10.1007/978-3-642-36359-7, Springer Heidelberg New York Dordrecht London. This essay was presented in a video conference at the Fringe 2013 in Nürtingen near Stuttgart on Sep. 11, 2013. However, the approach on holography presented by Joseph W. Goodmann does not constitute a teaching for generating two-beam interferograms, in particular also short-coherence interferograms, required for one-shot two-beam interferometry or for optical coherence tomography, which usually can be evaluated numerically very quickly compared to holograms. Here, no teaching is provided for a structured illumination of the object in the light-field approach for two-beam interferometry.
In Stanford Tech Report CTSR 2005-02 [16] in image 1 on page 2, Ren Ng et al. describes light-field photography with a camera according to the plenoptic approach, which has a microlens array in the image plane. However, this assembly cannot be applied to one-shot two-beam interferometry and optical coherence tomography, since usually two-beam interferences cannot be generated with this assembly.
In the patent document U.S. Pat. No. 7,177,029 B2 [17], Peter J. deGroot describes a stroboscopic interferometer, which detects the interferogram data time-serially with a series of light pulses, i.e. based on the time domain method. Consequently, no one-shot measurement can be performed with this approach.
In the published patent application DE 10 2011 000 213 A1 [18], J. Niehues and Peter Lehmann describe an assembly for white-light interference microscopy, in which confocal illumination is produced by means of a spatial light modulator. With this approach, too, it is not possible to perform a one-shot measurement, since it is a time domain approach.